Harvard Study Reveals New Shark Skin Properties

Research under more natural conditions details unexpected findings.

There is growing interest in the way in which animals have developed particular characteristics to help them move more efficiently, with this field of study being described as biomechanics. Sharks as an example are aided in terms of their movement by their remarkable skin. With its sharp tooth-like scales known as denticles, this was believed to behave like the dimples on a golf ball, disturbing the flow of water over the surface to reduce the drag.

Flawed studies
But George Lauder from Harvard University was not convinced by this explanation, bearing in mind the way in which research had been conducted. ‘All of the shark skin studies were done on flat shark skin mimics that were held straight and immovable. But shark skin moves” explains Lauder. Working with masters student Johannes Oeffner, Lauder decided to investigate the fluid dynamics of shark skin and its analogues further, in a more natural situation. The aim of the duo was to discover how the way in which the fish moves actually impacts on the water as it comes into contact with this rough surface.

Oeffner therefore took skin sections from a mako shark purchased in a Boston market, and attached this to both sides of rigid aluminium foil. He immersed the foil in a flow tank, and replicated the shark’s swimming motion by wiggling it from side to side. He was then able to measure the rigid foil’s speed as it moved, by matching it with the flow of water moving in the opposite direction.

Having recorded the foil’s swimming speeds with intact skin – complete with denticles – Oeffner carefully sanded off the denticles and set the foil swimming again. Now though, rather than slowing down – as the researchers expected – the denticle-free foil speeded up. So the shark skin’s denticle surface impeded the rigid swimmer. “But then we remembered our premise that the sharks aren’t rigid” said Lauder. “So how would the shark skin perform when flexing like a real fish?”

Variable performances
Gluing two pieces of shark skin together to produce a flexible foil, Oeffner then repeated the swimming experiment, and this time, the denticles had a dramatic effect. The intact skin foil swam 12.3% faster than the sanded skin. The shark’s rough surface improved the swimming performance spectacularly.

However, when the scientists tested the swimming performance of two shark skin mimics – a sharp-edged riblet design and the famous Speedo® Fastskin® FS II fabric – they were in for a shock. Although the riblet surface improved the flexible foil’s swimming speed by 7.2%, the dented surface of the Speedo® fabric had no effect at all. Lauder points out that figure-hugging Fastskin® swimming costumes probably enhance the swimmer’s performance in other ways.

After proving that the denticles on shark skin significantly improve the fish’s propulsion, Lauder and Oeffner were keen to find out how they…